The method traditionally employed for incorporating additives into polymers involved first polymerizing the monomer and then adding stabilizers to the polymer by extrusion or other melt mixing techniques, thereby producing pellets. Stabilizers have traditionally not been added into the polymerization reactor because neat stabilizers usually deactivate the polymerization catalyst and terminate the polymerization reaction. Disadvantages of the current melt mixing techniques include an appreciable downtime for cleaning of the blending equipment, heat sensitivity of some additives, and pelletization problems, particularly with very high melt flow (&gt;100 g/10 min) polypropylene resins.
A variety of patents teach methods for blending additives with polymers. One such patent is U.S. Pat. No. 3,591,409. Patent '409 teaches a method for preparing a polymer composition by high intensity blending of a mixture of particulate thermoplastic resin, hydrocarbon wax, and particulate solid additive at high temperatures. This teaching requires that all ingredients be placed in the mixer at once, without the separate pretreatment of any one of the components of the mixture.
In contrast with this conventional teaching, it has been discovered that by adding discrete particles to the reactor during polymerization (wherein the particles consist of a core of additive surrounded by a polymer wax), good yield of stabilized polymer can result. A variety of patents teach methods for making discrete particles or capsules containing different kinds of fill materials. U.S. Pat. No. 4,306,993 to Minnesota Mining & Manufacturing Company discloses a method for making microcapsules which can be used to tag books, paper currency and especially explosives and explosive devices. Other discrete particle preparation techniques are disclosed in the following patents: DT 2,160,852 which discloses microcapsules containing wetting agents, and U.S. Pat. No. 4,224,422 which discloses a curable epoxy resin system useful as an adhesive, wherein the curable one part system comprises a rupturable impermeable microcapsule having shell walls of a crosslinked interfacial polyurethane-polyether reaction product of an aromatic polyisocyanate. U.S. Pat. No. 4,382,326 describes acrylate copolymer microsphere structures using glycerol and cycloaliphatic polyepoxides filled with a liquid Lewis acid-glycerol complex. Tacky spherical acrylate copolymer particles are disclosed in Minnesota Mining & Manufacturing Company's U.S. Pat. No. 4,098,945. Patent '945 teaches a liquid-filled capsule having a microporous capsule wall made of a dispersed solid-phase and a continuous phase, wherein the capsule forms a single phase in the liquid state but separate phases when cooled to solid, and wherein the volume contraction ratio is greater than or equal to 1.2. The preferred capsules are spherical and the wall is made of the solid, semi-crystalline polyolefin, an amorphous hydrocarbon resin and an at least partially crystalline wax capable of separating out as a discontinuous mass.
Liquid-filled capsules for controlled release of fertilizers having a microporous wall of semi-crystalline polyolefin, hydrocarbon resin and a wax are disclosed in U.S. Pat. No. 4,002,458. U.S. Pat. No. 3,985,840 describes microspheres prepared by forming a homogeneous molten liquid phase using a solid semi-crystalline polyolefin, a hydrocarbon resin and at least partially crystalline wax, and wherein the liquid phase is allowed to cool and the wax separates out as a discontinuous mass dispersed throughout the matrix. U.S. Pat. No. 3,977,992 discloses a controlled release capsule prepared from 2% wax, 5% hydrocarbon resin and the remainder of semi-crystalline polyolefin such as polyethylene.
Since it has long been desired to find a method of cheaply blending stabilizers, antioxidants and other additives into polymers in the reactor without compounding or extruding, such that these additives are dispersed throughout the polymer, a novel invention has been developed. Since it has been desired to achieve a method of blending additives into polymers, without resorting to a process requiring the use of solvents, the present invention was developed. This invention relates to the preparation of coated additives that can act as antioxidant and/or stabilizer and which can be incorporated into a polymer during polymerization on short notice and with good efficiency.
This invention is an improvement over my prior invention described in Ser. No. 147,953, filed Jan. 25, 1988, now U.S. Pat. No. 4,853,426 as it extends the technology of coating additives to low melting additives. Both high melting and low melting additives can be used according to this invention. In my prior invention molten polyolefin wax is sprayed onto a fluidized bed of solid additives. For relatively low melting additives this method of coating will not be operable because the additives will melt in the bed. This limitation has now been overcome via use of an aqueous emulsion of polyolefin which will be sprayed onto the fluidized bed of additives. The water rapidly evaporates in the fluidized bed leaving a coating of polyolefin on the additives. The coated additive particles can then be added to a olefin polymerization reactor producing polymer which already has the additives incorporated. This new invention represents a lower temperature coating operation and thereby is lower in energy cost compared to my prior invention.
The production of additive-containing polymer directly from the reactor will eliminate the extrusion pelletization step currently used to incorporate additives. This reduced thermal history will increase the oxidative stability of the polymer.